Berin passes along this list of the top 20 geek novels. I've read The HitchHiker's Guide to the Galaxy, 1984, Brave New World, Dune, I, Robot, Foundation, Snow Crash, Watchmen, Cryptonomicon, Consider Phlebas, Stranger in a Strange Land, American Gods, and The Diamond Age.
American Gods is not Gaiman's best; I prefer Good Omens or Neverwhere. Similarly, I enjoyed Time Enough For Love more than Stranger in a Strange Land, although the latter clearly had more cultural influence. Adams never topped the HHGTTG, and what I read of his Dirk Gently series was a seriously underwhelming experience. I'd never even heard of Trouble With Lichen; aren't The Chrysalids and The Day of the Triffids much more well known?
Unsurprisingly, there are no female authors on the list; given the group, I might have expected Le Guin.